1. Field of the Invention
The present invention relates to improvements in the control of friction and inertia welding processes. More specifically, the invention concerns an improved apparatus for friction and inertia bonding.
2. Description of Related Art
Inertia welding is a known process for joining two metallic components and comprises fastening one component to and coaxially with a flywheel, driving the flywheel up to a given speed of revolution, thereby generating a given value of energy, and forcing the rotating component against the static component after removing the drive from the flywheel. The energy stored in the flywheel continues to rotate the component and the resultant friction between the relatively rotating components in turn generates sufficient heat, in the first instance to soften the interface and secondly to assist the pressure applied to achieve a solid phase weld.
A friction welding process differs from inertia welding in that the rotary drive is powered, rather than being derived from stored energy, and the rotation may be stopped by means of a brake rather than by dissipation of the stored energy.
During both processes, metal is displaced or "upset" which results in a shortening in the overall axial length of the two components. The total amount by which the length of the two components is reduced may be termed the "upset length". In friction welding, at least, this can be subdivided into an initial "burn off" or "friction" length (a reduction in length caused during a first period of time when heat is being generated to soften the interface) and a "forge length" (a subsequent reduction in length while the applied pressure forges the two components together).
Commonly, the applied pressure is increased after the burn-off length has been achieved, so that the forging pressure is greater than the friction pressure. In inertia welding, there is generally less distinction between these two phases of the welding process. A discussion of such matters may be found in British Standard BS 6223: 1982, published by the British Standards Institution.
UK Patent GB No. 1254022 is an example of friction welding apparatus in which the speed of the drive motor is controlled in accordance with a pre-programmed speed-time relationship. Such techniques are useful for controlling the quality of the resulting weld. However, such techniques give no control over the resulting amount of upset metal, and in particular give no control over the upset length. For example, in friction welding of two typical components by a conventional method, there might be a tolerance of .+-.0.5 mm in the upset length. Moreover, speed control as shown in GB No. 1254022 is not possible in inertia welding, since it is impossible closely to control the speed of the massive flywheel used, owing to its inertia. In inertia welding of two typical components by a conventional method, since there is no control of the initial burn-off length, there might be a tolerance of .+-.1 mm in the upset length.
Because there is such a tolerance in the upset length, critical components need to be designed oversize, and a large amount of expensive machining to size is needed after the welding operation. Furthermore, in some applications such as disc-to-disc welding of rotor disc assemblies in gas turbine engines, subsequent machining to size may be impractical or impossible because of the complicated shapes of the components. Accordingly, conventional inertia and friction welding processes are not feasible for such applications.
UK Patent GB No. 1293141 relates to a method of controlling friction weld quality, and states that the rate of the initial burn-off is of more importance than the actual burn-off length. Accordingly, this patent proposes monitoring the burn-off rate (the rate of change of length with respect to time during the initial burn-off period). The burn-off rate is compared with a pre-set reference value and the axial welding pressure is adjusted to keep the burn-off characteristic following a desired straight line. This method cannot control the resulting overall upset length, for two reasons. Firstly, only the rate of metal upset is controlled, for weld quality reasons. This gives no direct control over upset length, and the patent is not concerned with controlling upset length. Secondly, the control is only during the burn-off phase, and there is no suggestion of control during the forging phase, which has an important effect on total upset length.
Another inertia welding process is shown in UK Patent GB No. 1439277. In this patent, in order to provide assurance of the quality of the weld, the pressure, speed and upset are monitored throughout the welding process. Should the pressure, speed or upset stray outside predetermined ranges within which a good quality weld can be assured, an indication of this fact is provided, and the welding process may be automatically stopped. There is no feedback control over the welding parameters. Such a quality control method is only acceptable because in practice the predetermined ranges within which the parameters can vary while still producing a good quality weld are quite wide. Were this not so, a large proportion of the components welded would need to be rejected. For example, ranges of .+-.7% permissible variation in the nominal, "ideal" weld parameters would be common. The problem with which the present invention is concerned, however, is control of the total upset length in situations where such variation would be intolerable.